1. Field of the Invention
The invention relates to a process for the enzymatic preparation of enantiopure 1,3-dioxolan-4-one and 1,3-oxathiolan-5-one derivatives.
2. The Prior Art
Enantiopure derivatives are used as starting materials and intermediates in the synthesis of agrochemicals and pharmaceuticals. Many of these compounds are currently prepared and marketed as racemate or mixture of diastereomers. However, in many cases, the desired physiological effect is brought about by only one enantiomer/diastereomer. The other isomer is, in the most favorable case, inactive, but it may also counteract the desired effect or even be toxic. Processes for separating racemates are therefore of increasing importance for the preparation of compounds of high enantiopurity.
It is known that racemates of chiral compounds can be separated with the aid of enzymes. A large number of publications describe enzymatic kinetic resolutions of racemates of esters with lipases and esterases. However, to date, there is no process which permits the simple separation of 1,3-dioxolan-4-one and 1,3-oxathiolan-5-one derivatives. Enantiopure 1,3-dioxolan-4-ones are of great interest for the preparation of compounds with antiviral activity, such as, for example, the 1,3-dioxolanyl-nucleoside “dioxolane-T” (NB=thymine in equation 1) and similar structures (Bioorg. Med. Chem. Lett. 1993, 3(2), pp. 169-174). Equation 1
For preparing enantiopure 1,3-dioxolanyl-nucleosides, the separation into the enantiomers has to date been carried out at the considerably more costly nucleoside stage. This process was described for the first time by L. J. Wilson et al. (Bioorg. Med. Chem. Lett. 1993, 3(2), pp. 169-174). The butyric ester of the primary hydroxyl group of a 1,3-dioxolanyl-nucleoside is in this case hydrolyzed with the aid of pig liver esterase, and the two pure enantiomers are thus obtained in good optical yields. WO 00/22157 (inventors: Yao, Y. et al.) describes a variant of this process by resolution in nonhomogeneous systems.
Enantiopure 1,3-oxathiolan-5-ones are likewise of great interest for the preparation of compounds with antiviral activity, such as, for example, the 1,3-oxathiolanyl-nucleoside Coviracil® (also Emtricitabine, formerly FTC, 4-amino-5-fluoro-1-[(2R, 5S)-2-(hydroxymethyl)-1,3-oxathiolan-5-yl]-2(1H)-pyrimidinone; equation 2) and similar structures (J. Org. Chem. 1992, 57(21), pp. 5563-5565, WO 91/11186, WO 92/14743, WO 00/22157). Equation 2
For preparing enantiopure 1,3-oxathiolanyl-nucleosides it is possible for the separation into the enantiomers to take place at various stages. Thus, an enzymatic racemate resolution is possible at the oxathiolanone stage. This is described by Liotta et al. (WO 91/11186). In this case, the stereoselection is achieved by enzymatic cleavage of an ester substituent in position 2 of the oxathiolane ring (equation 3). Equation 3
An example which is mentioned is the hydrolysis of the butyric ester (R=C3H7) in the presence of pig liver esterase (PLE).
The second possibility is racemate resolution at the considerably more costly nucleoside stage. This process is described by Liotta et al. (J. Org. Chem. 1992, 57(21), pp. 5563-5565 and WO 92/14743). In this case, various ester acyl groups of the nucleoside racemate are stereoselectively eliminated in the presence of lipases or proteases (equation 4). Equation 4
In some cases, high enantiomeric excesses (ee(ester)>98%) are achieved with good yields (y(ester)−45%).
An improvement of the process of WO 92/14743 is to be found in WO 00/22157 (inventors: Yao, Y. et al.) through the use of non-homogeneous reaction systems (addition of water-immiscible cosolvents) for resolution of racemates of oxathiolanyl-nucleosides.
These processes of racemate resolution at a very late stage entail the serious disadvantage of unnecessary use of materials and long plant usage, because the maximum yield of a racemate resolution is 50%. The remaining 50% (the compound with the wrong handedness) is usually discarded.